Digestion and Absorption

Photo by: olly

Digestion is the breakdown of food into smaller particles or individual
nutrients
. It is accomplished through six basic processes, with the help of several
body fluids—particularly digestive juices that are made up of
compounds such as saliva, mucus,
enzymes
, hydrochloric acid, bicarbonate, and
bile
.

The six processes of digestion involve: (1) the movement of food and
liquids; (2) the lubrication of food with bodily secretions; (3) the
mechanical breakdown of
carbohydrates
, fats, and
proteins
; (4) the reabsorption of nutrients—especially water; (5) the
production of nutrients such as vitamin K and
biotin
by friendly
bacteria
; and (6) the excretion of waste products. Comprehension of the tasks or
processes needed to break down food are essential to an understanding of
how and when food really begins to function within the body. For example,
not understanding that carbohydrates break down into
glucose
could lead one to believe that the best source of glucose is in liquid
form such as a soft drink. This could cause one to miss out on the
nutrients (and great taste) in fruits, vegetables, and grains. Likewise,
not understanding the digestion process could lead a person to believe in
the myth of "food combining," or perhaps to think it is
normal to be hungry all the time. But, in fact, the digestive processes
normal to human
physiology
can simultaneously handle carbohydrates, fats, and proteins—and
allow people to go several hours between meals, especially if meals are
balanced in
fiber
and the individual nutrients needed.

GI Tract Physiology

Digestion begins in the mouth with the action of salivary amylase. The
food material then progresses past the esophagus and into the stomach. A
bolus (soft mass) of chewed food moves by muscular wave actions, called
peristalsis,
from the mouth to the pharynx, and then past the epiglottis that covers
the larynx. The epiglottis closes off the air passage so that one
doesn't choke. The cardiac sphincter prevents reflux of stomach
contents into the esophagus.

From the Stomach to the Small Intestine

Food mixtures leaving the stomach are called
chyme,
and this empties into the small intestine after about two to four hours
in the stomach. The small
intestine is where most digestion takes place. A pyloric sphincter
controls the rate of flow of chyme from the stomach into the small
intestine.

Most digestion occurs in the upper portion of the small intestine, called
the
duodenum.
Below the duodenum is the
jejunum,
and then there is the last segment, called the
ileum.
About 5 percent of undigested food products are broken down in the ileum.
This is why some people can have a small part of their intestine removed
and still seem to digest most foods with little problem.

Digestion of food that enters the small intestine is usually complete
after three to ten hours. Once digestion is essentially finished, waste
products leave the ileum with the help of fiber, and these solids then
enter the large intestine (the colon). In the colon, water is reabsorbed;
some nutrients are produced by friendly bacteria (vitamin K, biotin,
vitamin B
12
); fibers are digested to various acids and gases; and
minerals
, such as potassium and sodium, are reabsorbed (when needed). Any fiber
that is not broken down—and small amounts of other undigested
products—are excreted in the feces.

Protective Factors

During digestion in the stomach, large proteins break down into smaller
protein forms, and harmful bacteria can become inactive. Hydrochloric acid
is especially important for this because it lowers the
pH
of the stomach contents below 2. Along with the uncoiling of protein in
the stomach, a little carbohydrate and lipid are broken down with the help
of enzymes (called
amylase
and
lipase,
respectively).

In the stomach, carbohydrates in foods turn to starch, but it is not until
the chyme reaches the small intestine and becomes more neutralized that
starch turns to simple sugars that are then absorbed into the portal vein,
which transports them to the liver. Also in the small intestine, lipids
(mostly in the form of
triglycerides
) are emulsified and form
monoglycerides
and free
fatty acids
that can then go through the
lymph system
to the heart and bloodstream.

As previously mentioned, the mouth, stomach, small intestine, and colon
are the major organs of digestion. However, the liver, gallbladder, and
pancreas are also important to the process. The liver detoxifies foreign
compounds, such as natural
toxicants
in foods and
drugs
. The liver also makes bile, an emulsifier, which enters the small
intestine and prepares fats and oils for digestion. This bile is stored in
the gallbladder prior to delivery to the small intestine. A
hormone
called
cholecystokinin
helps control the release of bile.

The pancreas makes pancreatic juice consisting of enzymes (amylases,
lipases, and proteases) and bicarbonate, which helps neutralize acidic
secretions produced during digestion. The pancreas delivers the pancreatic
juice to the small intestine, in response to a signal of food in the
intestine and the release of the hormone
secretin.
The pancreas also has another function, the secretion of the hormones
insulin
and
glucagon,
which helps maintain a steady state of blood sugar in the body (insulin
decreases blood glucose concentration, while glucagon increases it).

Food moves from the mouth to the epiglottis, bypassing the trachea, into
the esophagus, past the cardiac sphincter into the stomach, past the
pyloric valve into the small intestine (duodenum, jejunum, ileum), and
then

The tract running from the esophagus to the large intestine is called
the alimentary canal, and it is where most digestion occurs. As food
is pushed through the system, it encounters numerous specialized
processes that act on it in different ways, extracting nutrients and
rejecting waste.

[Illustration by Argosy. The Gale Group.]

past the ileocecal valve into the colon. Waste then leaves the colon
through the rectum and anus. When chyme reaches the small intestine, the
pancreas and liver contribute to the digestion by providing products such
as bicarbonate, enzymes, and bile.

Absorption

Absorption is the movement of
molecules
across the
gastrointestinal
(GI) tract into the circulatory system. Most of the end-products of
digestion, along with
vitamins
, minerals, and water, are absorbed in the small intestinal lumen by four
mechanisms for absorption: (1) active transport, (2) passive diffusion,
(3) endocytosis, and (4) facilitative diffusion. Active transport requires
energy
.

Nutrient absorption is efficient because the GI tract is folded with
several surfaces for absorption and these surfaces are lined with villi
(hairlike projections) and microvilli cells. As one nutrition textbook
puts it, each person has a surface area "equivalent to the surface
of a tennis court" packed into his or her gut (Insel et al., p.
81). Efficient absorption can be compromised due to
lactose intolerance
. Lactose intolerance is not uncommon in the world, affecting about 25
percent of the U.S. population and 75 percent of the worldwide population.
It is usually due to the lack or absence of the enzyme
lactase,
which breaks down milk sugar.

Lactose intolerance is not a food
allergy
. Food allergies are serious, even life threatening, but most people with
lactose intolerance can digest small amounts of milk, especially in yogurt
and cheese.

Protein, carbohydrate, lipid, and most vitamin absorption occur in the
small intestine. Once proteins are broken down by proteases they are
absorbed as dipeptides, tripeptides, and individual
amino acids
. Carbohydrates, including both sugar and starch molecules, are broken
down by enzymes in the intestine to disaccharides called
sucrose, lactose,
and
maltose,
and then finally into the end-products known as
glucose, fructose,
and
galactose,
which are absorbed mostly by active transport. Lipase, an enzyme in the
pancreas and the small intestine, and bile from the liver, break down
lipids
into fatty acids and monglycerides; these end-products then are absorbed
through villi cells as triglycerides.

Alcohol is not a nutrient, but 80 percent of consumed alcohol is absorbed
in the small intestine. The other 20 percent is absorbed into the stomach.
Alcohol is absorbed by simple diffusion, which explains why
gastric ulcers
are not uncommon in people who drink excessively.

Coordination and Transport of Nutrients into the Blood or to the Heart

Hormones and the
nervous system
coordinate digestion and absorption. The presence of food, or the thought
or smell of food, can cause a positive response from these systems.
Factors that can inhibit digestion include
stress
, cold foods, and bacteria.

After foods are digested and nutrients are absorbed, they are transported
to specific places throughout the body.
Water-soluble
nutrients leave the GI tract in the blood and travel via the portal vein,
first to the liver and then to the heart. Unlike the vascular system for
water-soluble nutrients, the
lymphatic system
has no pump for fat-soluble nutrients; instead, these nutrients
eventually enter the vascular system, though they bypass the activity of
the liver at first.